This invention relates to a symbol indication drum adapted for use with apparatus requiring symbol indication, e.g. time measuring instruments or computers.
Symbols may be provided on the outer peripheral surface of a symbol indication drum directly by superposing symbols thereon by printing or etching, or indirectly by affixing to a drum body a belt on which symbols are formed beforehand. In either case, it is not possible to give accurate information by symbol indication unless the symbols are all disposed in predetermined positions with a high degree of precision.
When the symbols are superposed on the drum by printing, for example, it often happens that the master plate used slips and causes the symbols to be inordinately extended longitudinally or displaced transversely, with the result that the symbols are not correctly spaced apart from one another. When a symbol carrying belt is affixed to the drum body, expansion or contraction of the belt may be uneven throughout the length of the belt, thereby causing the symbols to be disposed at irregular intervals.
In testing whether or not the symbols are disposed in correct positions when a symbol carrying belt is affixed to a drum, there has hitherto been no alternative but to study the position of the outlines of each symbol. This has raised the problem of how to overcome the difficulty experienced in conducting the test.